1. Field of the Invention
The present invention is directed to devices and methods for determining, monitoring, and providing an indication of the state of charge of an object, such as a battery, using detectable magnetic flux, the object being useful in a variety of application, including a battery useful in automotive passenger vehicles.
2. Description of the Related Art
A battery consists of one or more cells connected in a series and/or parallel arrangement that chemically store electrical charge potential (energy) and deliver the charge at a pre-determined voltage when demanded by an external electric circuit load. Each of the battery cells contain two half-cells connected in series by an electrolyte. An electrolyte consists of anions (i.e., negatively-charged ions) and cations (i.e., positively-charged ions). One of the half-cells contains some of the electrolyte and an anode (i.e., negative) electrode toward which anions migrate. The other half-cell contains some of the electrolyte and a cathode (i.e., positive) electrode. The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either a solid or a liquid.
During battery operation, a redox (reduction—oxidation) reaction powers the battery. That is, the cations in the electrolyte are reduced (i.e., by the addition of electrons) at the cathode, and the anions are oxidized (i.e., by the removal of electrons) at the anode. As a battery cell discharges, ions flow from the anode, through the electrolyte, to the cathode. During charging, the ions then flow from the cathode, through the electrolyte, to the anode.
A theoretically perfect battery is capable of storing a charge that is a function of its design parameters and materials, delivering the charge to an external electrical load, and then being fully recharged to its original capacity. Thus, if one were to measure the charge, or amp hours, going into the cells during the charge cycle, and subtract from that accumulated charge the total charge going out of the cell during the discharge cycle, the resulting value would be an accurate indicator of the charge state, or how much energy is stored within the cell.
However, because the charge is stored chemically, each charge-discharge cycle (as well as normal temperature cycling, vibration, shock etc.) results in irreversible changes within the individual cells. Moreover, the rate of charge and/or discharge can also manifest in changes to the cell capacity. The common result of these changes is that less energy is stored during each subsequent charge cycle. For example, as the number of charge discharge cycles increases, the capacity of the cell decreases, such that at full voltage the cell may only be at 60% capacity rather than the 95% capacity when placed into service initially. Thus, the aforementioned method of subtracting the amount of charge used from the amount initially placed in the cell to determine the charge state is flawed because the actual charge capacity of the cell is reduced over time and actual usage at an unknown rate.
While knowing the actual and changing charge state of battery cells is needed, this knowledge has been primarily related to convenience in the use of portable electronic devices such computers and the like. The risk of uncertainty of the actual state of battery charge is almost always that an inconvenience will result if the battery charge falls below useful levels.
More recently, batteries have been used in conjunction with internal combustion engines to power vehicles. These so-called hybrid vehicles are capable of operating on battery power until such time that the battery is incapable of providing the mechanical energy demanded by the operator, at which point the internal combustion engine, through an electrical generator, either supplants or augments the available battery charge.
There is increased interest in the use of vehicles that operate solely by battery power, eliminating the internal combustion engine and generator altogether. This trend is facilitated in part by advances in chemical battery technology. However, regardless of the technology in which a batter is used, it is still necessary to actively determine, monitor, and provide the battery state of charge information. In purely electric vehicles, this need rises beyond convenience because there is no motive backup as is the case with hybrid vehicle configurations.
Attempts to compensate for the problems with prior techniques to determine charge utilize algorithms that take into account a variety of factors including the amount of charge/discharge cycles, the rate of charge/discharge, and other factors in an effort to weigh the integrated sum of the charge going into the cell so that the measured result corresponds to the percentage of actual cell capacity available versus the percentage of theoretical cell capacity.
Other known methods include measuring voltage and/or current at one or more locations within the discharge path. This method is prone to numerous errors. As with the technique of measuring “charge in vs. charge out”, this method relies on the assumptions that either the properties of the cell remain constant (which is not true in practice), or that the changes over time and usage reliably follow an algorithm correction factor (which is generally the case, but only within a statistical band, and with potential “outliers”). Because a battery is comprised of numerous cells connected in a series and/or parallel configuration, an individual cell that is behaving as a statistical outlier with reference to the other cells can significantly degrade the function of the battery as a whole.
In U.S. Pat. No. 5,537,042, a single coil located in proximity of a cell electrode is excited with an alternating electric current. The cell charge state determines the complex impedance of the coil and its near environment. There are a number of limitations to this approach. For example, the measurement circuit output is not solely reflective of the complex impedance of the coil/electrode circuit itself, but of the entire circuit, including all of the electrical interconnects and wire runs between the coil and measuring circuit. Any resistive or reactive influence to the circuit will add error to the measurement. Moreover, the '042 patent teaches embodiments in which the complex impedance measurement of interest relates to the “plates” of the battery cells. Accordingly, the disclosed embodiments of such a device will employ relatively large coils in order to maximize their interaction with the cell plates. In addition to the obvious packaging, weight, and manufacturing issues with such an arrangement, larger coils require substantially more power for excitation at the requisite levels, leading to less efficiency and increased likelihood of propagating electromagnetic fields that may interact with other measurement coils or nearby electronics.
Other methods of evaluating the state of charge within a battery cell require that a sensor be immersed within the cell, or that a sample of the cell electrolyte be removed, in order to measure properties such as density (specific gravity) or optical characteristics.
Accordingly, what is needed is an apparatus and method for determining, monitoring, and providing an indication of the state of charge of a battery that overcomes the limitations of the prior art.